Conventionally, β-amylases have been known as those derived from plants such as soybean, wheat, barley, malt, sweet potato and potato. Particularly, β-amylases which are extracted and purified from cereals such as soybean, wheat, barley, and malt have been widely industrially used for producing maltose-containing syrups used in the sugar production industry, bread-making industry, and brewing industry. Among β-amylases derived from plants, one derived from soybean has high enzyme activity and high thermostability.
By the way, the price of corn has been increased due to demand expansion of bioethanol in recent years. According to this reason, planting has been shifted from soybean and wheat to corn. Therefore, soybean, wheat, barley, and the like become scarce and prices thereof are increased, and thus, securing materials of β-amylases is in a difficult situation.
β-amylase is an enzyme that acts on polysaccharides having α-1,4 bonds of glucose as main chains, such as starch and glycogen, and digests into maltose units from non-reducing terminals. β-amylase has been known for its existence in higher plants such as soybean and wheat from a long time ago. In 1972, since existence of an enzyme showing an action mechanism that is the same as a higher plant β-amylase also in a microorganism was disclosed, many microorganisms have been found as β-amylase producing bacteria (Non-patent Document 1).
So far, bacteria belonging to Bacillus sp. such as Bacillus cereus, Bacillus polymyxa, Bacillus circulans, Bacillus megaterium and Bacillus stearothermophilus, or Streptomyces sp., Pseudomonas sp., and the like have been reported as β-amylase producing bacteria. However, many of the bacteria have low productivity and hardly attained practical applications.
On the other hand, an amylase produced from filamentous fungi such as Aspergillus sp. is an endo-type enzyme which digests amylose and amylopectin. Therefore, when the amylase is used, glucose, maltotriose, and other oligosaccharides have been also largely produced in addition to maltose. What is more, such an amylase has low thermostability and its practical use in maltose production is small.
Bacillus stearothermophilus produces a maltose-generating enzyme having high thermostability (Patent Document 1, Non-patent Document 2). This enzyme is an exo-type and generates maltose from a non-reducing terminal of starch, and the generated α-maltose. In addition, the enzyme does not exactly hydrolyze in a maltose unit as a plant-derived β-amylase dose, and it has been reported that the enzyme also generates small amounts of maltopentaose (G5) and maltohexaose (G6) in addition to maltotetraose (G4), maltotriose (G3) and maltose (G2) in an early stage of a reaction, digests Shardinger dextrin into maltose and glucose, and digests maltotriose into maltose and glucose. Accordingly, 6 to 8% of glucose is contained in a digestion product of starch by this enzyme. Therefore, the enzyme is not suitable for production of a maltose syrup with high purity.